Metal-air cells typically include a metal anode, an air cathode, and a separator all disposed and supported in some sort of container. The metal anode usually comprises a fine-grained metal powder, such as zinc, aluminum, or magnesium, which is blended together with an aqueous electrolyte, such as potassium hydroxide, and a gelling agent into a paste. The separator is a porous material that allows the passage of electrolyte between the cathode and anode, but prevents direct electrical contact therebetween and short circuiting of the cell.
The air cathode is a catalytic structure designed to facilitate the reduction of oxygen. Typically, it is composed of active carbon, a binder, and a catalyst which, together with a metal current collector, are formed into a thin sheet. The air cathode also commonly incorporates a hydrophobic polymer, such as polytetrafluoroethylene or polystyrene, directly into the cathode sheet and sometimes also as a coextensive film. The hydrophobic polymer prevents electrolyte from flooding the cathode or passing through it and leaking from the cell. The container includes oxygen access openings, diffusion chambers and the like which are designed to allow sufficient oxygen to reach all parts of the air cathode.
Metal-air cells have high specific energies. In fact, zinc-air cells have the highest specific energy, up to 450 Wh/kg, of all aqueous primary systems, and high energy per unit volume as well. The components of zinc-air cells also are relatively benign.
Because of their high energy density, button cells incorporating zinc-air chemistry are the most popular batteries for hearing aids. The much larger majority of electronic devices, however, has higher energy requirements requiring the use of larger (i.e., greater than one ampere-hour capacity) cells or batteries. Despite the electrochemical advantages of metal-air and especially zinc-air systems, carbon-zinc and alkaline manganese dioxide systems continue to dominate the much larger world market for larger primary batteries.
Many portable electronic devices, such as portable computers, also place severe constraints on battery weight and volume. In such applications, prismatic cells would be preferable over button or cylindrical cells, which latter type of cells, in general, require more space to be allocated in the device than the cells themselves actually occupy. Prismatic cells also can be much thinner than alkaline cells of equivalent capacity.
Attempts to scale up and reconfigure zinc-air button cells to a larger, prismatic configuration, however, have generally failed. Zinc-air batteries currently are not a competitive option for use in the full spectrum of consumer and electrical products, and they represent a small portion of all primary batteries sold today.
A major problem has been in achieving an inexpensive, light-weight and easily constructed prismatic configuration which is leak-proof, but which provides for efficient electrochemical discharge of the cell. The design of conventional zinc-air button cells and existing manufacturing procedures have not solved the problems for prismatic cells.
A further and significant problem concerns designing a prismatic zinc-air cell that would be amenable to assembly for mass production.
It is important, but sometimes difficult, to prevent electrolyte and anode paste from contacting sealing surfaces. Virtually all commercial zinc-air button cells utilize a multicomponent container, at least one part or subassembly of which is shaped substantially like a cup. A preconstituted anode paste may be filled into the cup. Alternately, a dry mix of zinc and gelling agent may be poured into the cup, after which aqueous electrolyte is added. In either event, the process of loading the cup can be relatively messy. Paste or electrolyte may be spilled onto the sealing surface of the cup in the loading process.
Also, at times too much anode material may be loaded into the cup, and the excess material squeezed into sealing areas when the container is assembled. If anode paste or electrolyte contaminates container sealing surfaces through such processing accidents or any other mechanism, it can cause imperfections in the seal through which leaks may occur. In larger cells, the area which must be sealed is correspondingly larger, and thus, the likelihood of defects occurring in the seal is correspondingly greater.
It also is important to control relatively precisely the amount of gelled zinc anode paste which is incorporated into a cell. By doing so, costs may be controlled more closely; and cells having more uniform discharge capacities will be produced. As noted, if too much anode material is loaded into a cell container, the excess anode material also may be squeezed into sealing areas when the container is assembled.
If the container is constructed from plastic materials, however, problems may be created if too little anode material is dispensed into the container. Most commercial zinc-air cells have conductive, metal containers, and the gelled zinc anode paste is in intimate contact with the container. Plastic is nonconductive, and so zinc-air cells with plastic containers must incorporate an anode current collector. Such collectors often are in the form of a thin metal sheet or wire grid, and such current collectors should be in intimate electrical contact with the anode to the greatest extent possible. If too little gelled zinc anode paste is dispensed into the cell, electrical contact between the current collector and the anode may be impaired.
It seems apparent that, for considerable time, there has existed a substantial need for larger prismatic metal-air cells, especially for thin prismatic zinc-air cells, which can satisfy the energy requirements for a wide variety of applications. It seems further apparent that a considerable amount of effort has been directed to providing suitable cells capable of providing satisfactory performance. Yet, despite recognition of the need and the considerable efforts made to date, there still exists the need for prismatic, and especially for thin prismatic metal-air cells such as zinc-air, which can provide satisfactory and reliable performance for a wide variety of commercial applications.
An object of this invention, therefore, is to provide a prismatic metal-air cell which provides more reliable electrochemical performance in service. A related and more specific object is to provide a thin prismatic zinc-air cell having satisfactorily reliable electrochemical performance in service.
It also is an object to provide a prismatic zinc-air cell having a gelled zinc anode which is more leak resistant. A related object is to provide such a cell wherein the container parts may be sealed together more reliably.
Another object of this invention is to provide a prismatic zinc-air cell having a gelled zinc anode in which the amount of gelled zinc anode paste dispensed into the cell is more precisely controlled.
It is a further object of this invention to provide prismatic zinc-air cells having a gelled zinc anode and a plastic container, wherein more reliable contact between the gelled zinc anode and the anode collector is provided.
Yet another object of this invention is to provide a prismatic zinc-air cell which is simple in design and easily and economically manufactured in mass production.
It also is an object of this invention to provide a method of fabricating prismatic zinc-air cells with a gelled zinc anode which produces more reliable sealing of container parts.
Another object is to provide such methods which control more precisely the amount of gelled zinc anode paste dispensed into a cell.
It is a further object of this invention to provide a method for fabricating a prismatic zinc-air cell with a gelled zinc anode and a plastic container which produces more reliable contact between the gelled zinc anode and the anode collector.
Yet another object of this invention is to provide a prismatic zinc-air cell with a gelled zinc anode and methods for fabricating such cells wherein all of the above-mentioned advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.